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MG Midget and Sprite Technical - Yet Another Clutch Puzzle

This one has me perplexed, and I'm hoping somebody will say, "oh - that old problem", and tell me what to do about it.

You may remember that my release bearing disintegrated a while ago, and with Bill's help (a gift of a roller bearing and fork) it seemed to be better than ever.

Well, it was.

Last weekend, on our way to an MGCC event,about 30 miles from home, I went to change down from 4th to 3rd for a roundabout, and it wouldn't go into any gear from the gate. At first I thought, "gearbox", but after slowing almost to a stop and re-clutching, it behaved normally.

The next time I wanted to change down, I double de-clutched, without a problem. So not gearbox. We got to the event, ddc-ing all the way.

Starting back, with about 60 miles to drive, it behaved for 10 minutes, then the pedal started to go soft, always after a longish run in top. I started gently pumping it up whenever I contemplated a gearchange, and it worked OK. Changing up through the gears - in rapid sequence - was always OK.

You know how you can feel what's happening with the release bearing through your clutch foot sometimes? Well I reckon it felt as if the fork was relaxing right back while I was in top, and when I pumped, I thought I could feel the bearing coming back to its correct angle against the bearing plate. That would have to mean that the slave piston was creeping back up the bore, or the fluid was creeping past it and filling up the boot.

This morning I had to swap garages, to put Little Blue in the one with light and power, which means driving half a mile. And right from the off, the clutch behaved perfectly. No soft pedal, and no drag during a very long reverse into the garage, a change into first to go forward, and a change into reverse again, all on the same depression of the clutch.

I'm off back down to the garage now, to get the car up in the air a bit, so I can see everything and check through the whole of the hydraulics, but when I come back to report findings, I'd be grateful if any of you Sherlocks has an explanation.
Nick

A real out there comment Nick but..

On the slave there is a rubber bung which is part of the piston. If you overextend the clutch travel then that can catch on the outside of the piston bore/cyl and prevent the piston from returning home.

Shown by little nicks on the side of the rubber...

I dont think that it sounds like your problem but it is a weird one....
Toby Anscombe

Thanks for replying, Toby. Unfortunately, the piston's going back only too well.

Reflecting on these symptoms, the only thing that's clear to me is that my clutch hydraulics are not a closed system as they should be. For the problem to right itself briefly, then build up again, there's a leak somewhere - of air, or fluid, or Watney's Red Barrel.
Nick

BTW - findings so far - no detectable fluid loss, no bubbles in the line, travel OK, little bit of slack at the master pushrod OK.
Nick

Hi Nick

sounds rather like a sluice-back into the master past the seals

as Toby suggests it could well be a result of slight over-moving of the seal in the master with subsequent surface scratching around the seal lip

bugger!

What fluid are you using?
(at risk of releasing far too many demons...)

Are you running the standard early type double master? I am sure you are, but maybe elderly seals are going the way of all flesh.
Bill

Make sure the vent hole in the master cylinder cap is not plugged. If the hole plugs, pressing the clutch will create a negative pressure environment leading to the slave cylinder slowly drawing the pushrod back during a long, vibrating ride. This sounds like what you are describing in your symptoms.
Mike G

Thanks Bill and Mike.

Good suggestions. I don't think the master's knackered yet (though it could be). I had it re-sleeved and assembled and pressure-tested by a shop.

The slave had a bit of an adventure when the release bearing went, and certainly over-reached. With all of the carbon and half of the carrier gone, it was bound to. Looked alright afterwards though.

I'll test the cap tomorrow.

One thing I found in my check of the connections this afternoon. The flexible pipe to the back of the slave was not absolutely tight. I managed to do it up about a flat and a half - maybe two flats. Would that be enough to leak?
Nick

could be

was there any sign of fluid out there

most likely under these circs is for air to get in maybe

Air seems to get in where surface tension stops fluids going

but when under pedal pressure I wouldn't bet on it

one thing that would worry me about the re-sleeving is that when fitted the sleeve would then need drilling through to allow the return fluid vent to operate (you know the two holes atop the bore that feed the cylinder and allow retracting.) If the sleeve was then not defraised at the puncture site any slight "edge" at the hole could damage the fine sealing edge on the rubbers

which could then allow the situation you describe to occur

sorta like the little piccy where the faint black marks indicate possible drilling fraise


Bill

Hi Nick

mine started doing that after i fitted the 5 speed box and roller bearing. I thought that with the roller bearing being heavier than the old one, it was drooping forward and hitting the clutch cover, which then pushed the bearing/fork away from the cover. An extra pump of the pedal restored it closer to the cover where it should be.

I lengthened the slave pushrod so it didn't push it too far away so i could still use the clutch with a single clutch depression.

I did try installing a spring on the end of the fork where it attaches to the pushrod to actually keep the roller bearing in contact with the cover at all times. However the bush in the release fork was quite worn and the bearing jangled too much when only in light contact with the cover, so I took the spring off. The pedal was much improved though, so I deduced the aforementioned pushing away to be the source of the problem.

Matt
Tarquin

Nick +
Hi, I'm not sure how midget/Sprite slave cyls. are set up but on an MGB the slave cyl. has a light spring inside between the end of the piston and the end of the cyl. bore which holds the piston out against the pushrod and fork just enough to keep the clearance to 0 at all times.
I have had MGB's that have rusted the spring and others that have simply just broken it .and the spring winds into itself and won't hold the piston out properly giving the exact same result as what you have. I've even had one brand new that didn't have a spring at all - very frustrating. I'd check to see if your car is supposed to have a spring and then if so check the condition of it. To me because your pedal returns to normal there isn't any air getting in the hydraulics, and also your master cyl appears to be holding pressure fine and it does absolutely nothing when driving along the problem you have is definatley related to the slave cyl creeping back. Cheers Willy
WilliamRevit

Thanks Matt and Willy. That sounds spot on. The roller bearing is pretty heavy, and it would certainly nod towards the bearing plate. And of course when the car's at rest, it isn't being gradually knocked back like it would be on a long run in top (I know you said all this, Matt, but I'm just thinking aloud).

I wonder why we managed the first 30 miles? Heat's the only variable I can think of.

Matt - how did you calculate the length of the pushrod? Piston right back up the bore and then let it go a little?

I'll check on the spring, Willy. Spridget slaves certainly should have one. The piston's very easy to push back up the bore, but it never occurred to me to look and see how long it took to come back - or indeed whether it came back at all.

It looks as if the slave will have to come off again, for me to be sure everything's right - I do so hate that top bolt, and I've just taken the car down off the ramps and stands. Hey ho.
Nick

Nothing too technical, just welding a 1" extension on, put it in to see how it fitted, then if it was too long ground a bit off. Easy!

My problem came and went too, 100's of miles without a problem, then it comes and goes for a few days.
Tarquin

Nick
- Clutch slave cylinder bolts. Shorten the top one to the minimum pitches of the thread needed to locate it, and rely on the longer bottom one to hold. Grind conical tips on both for easier fitting.

Regards,
Guy
Guy Weller

Matt - when you say:-

<<My problem came and went too, 100's of miles without a problem, then it comes and goes for a few days>>

. . . do you mean it still comes and goes after your fix?

Guy - sorry, I missed the last post. That's a great idea, and I shall get it done the next time the slave's off (which I still hope might not be this time, if Matt's solution works).

Isn't the working week a pain? And after a lie-in today, and a large breakfast, I'm only just starting the real-life side of my existence again.
Nick

To be honest it does sometimes come back. I think that without a strong spring in the slave, the release bearing will get pushed back from time to time. And I'll tell you something else, it's more common when driving on smooth roads. When it's rough it's fine. Maybe somethings sticking that the rough roads jiggle up and sort out.

My fix just makes sure that the clutch will work with a single pump, albeit right at the bottom of the pedal travel.

Working week, tell me about it! Only been at home an hour today so far!
Tarquin

some people still are on work...
Boohoo, i have no life anymore, im only working nowadays to keep the old midget going.




Oh, and something minor about a mortage orso. ;-)
Arie de Best

After last week's bleeding, and tightening the hose, I took her out for a 50 mile run today. Perfect (I mean really perfect) clutch for 30 miles, then a rapid deterioration; the last ten miles - in the London suburbs traffic - were awful.

I know this sounds drastic, but after making some careful observations (the ones I've missed making so far) of the action of the slave, and taking the slave off to look at its spring, seals and everything, I might just hoick the engine out again, and see what's what with the release bearing.

Agree about smooth roads, Tarks.

Bad luck, Arie. Good that you're able to play on the computer, though.
Nick

Your right Nick, this EVIL bbs has costed me enough production hours in the last couple years... but its so much fun :-)
Arie de Best

Nick

I dont want it to sound like I'm sticking up for the release bearing, but, aint nothing I can think of that would make the release bearing work the way you are describing

unless there is severe wear on the pivot bolt

The bush within the fork was changed when I had it and there was a lot of wear in the pivot bolt so I bought a new one, I bet I didnt send you that did I

:-(

Pedal pressure loss can only be hydraulic

pressure is supposed to be maintained within the closed system with no return loop as described

If the pressure action disappears (this is what you've been describing isnt it) it has to be a minor seep back from "front of seal" to "rear of seal" within the master cylinder

you wouldnt get signs of a leak because it would all stay inside the master, but oozing past the seal back into the reservoir section

I cant see from your recent comment whether you have checked for the spring inside the slave yet either...


We certainly get some good mysteries round here dont we
Bill

Sorry Bill - didn't see this post till this morning (frustration waiting for the page to open, in the last few days).

I finally got to take a day off and spend it in the garage yesterday.

If I push the pushrod back up the slave cylinder, it doesn't come back. This was clear when I started the car to drive it out of the garage five minutes later, because I had to pump the clutch several times to get a pedal.

I took the engine out and opened up, and there's nothing wrong with the assembly that I can see. By the way, the pivot isn't supposed to be stiff, is it? I assumed not when I was tightening it.

On the bearing plate there's a brown circle, matching the place where the release bearing would rest its weary head if it were slumped forward.

Another thing I noticed was that the newish boot is acting as a very effective spring, pushing the fork towards the back of the bellhousing. Hadn't thought of this before, but it feels stronger than the spring in the slave (which is there, by the way). I guess it would be nice if this boot held the fork in roughly the correct 'rest' position, but I don't think it should really have any role in the clutch release mechanism, and I've modified it with a scalpel.

I stuck a carbon release bearing back in - sorry to admit that, Bill, but my daughter Lizzy will be using the car for a couple of comps shortly, and she isn't the sort of girl to nurse it 30 miles home with a failing clutch. It might display the same symptoms, of course, and I'd like to get the MM one back in again, and I will, though I will have to find a way of uprating the return springing.

Then I put the engine back in, and by the time I was called home for tea I hadn't quite got everything back on, and had insufficient energy to lift the car to do the underneath bits, so I was glad of the full-time whistle.

I know I haven't addressed your master cylinder theory, but I'm not sure how to. If the slave piston is being forced back up its bore, where does the fluid go? Presumably past the master seal and back into the reservoir: same effect, different cause - or did I get that wrong? I don't really understand how the mc works at all.

Postscript:- in the silent watches of the night, it suddenly occurred to me to wonder what came first. What actually caused the release bearing to break up in the first place? Was it faulty, or was it nodding? Is there a hydraulic problem behind this? Must ask the missus how the clutch performed before that particular disaster. Might even admit to her that she wasn't responsible for breaking it, though that would mean the end of my present comfort.

Nick

Nick A problem that you strike now and then that causes the type of problem you have is caused by the the master or slave rubber boot not breathing as the piston goes back and forth. When the piston moves the air is compressed or lowered in pressure and can effect it on rebound. Drill a very small hole in the boots. They were made with a breather hole OE but many are supplied with out now. The air I refer to is around the push rod. Denis
DENIS4

Nick no need to say sorry

we'm mates aint we

In the circumstances I'd have done the same

I wonder if the fork/bearing interface gets hot in service and grips instead of swivelling? (where the clips slip over the boss of the carrier)

There are always lots of "eliminateable possibilities" to play with

getting the car back in use is the most important thing
Bill

Course we are, Bill. I just loved that big bugger with the bearings, even if it does nod off from time to time.

I will reassemble as it is, I think, and take it for another 30 mile run to see what happens. But I will make vent holes in the boots. Jolly good suggestion, Denis (Denis).
Nick

This thread was discussed between 20/09/2008 and 02/10/2008

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